Window assemblies for mounting in the walls of buildings such as homes, often include several different parts that must be fastened to each other and/or the building wall. The use of several parts that each must be fastened results in considerable cost for the manufacture and transportation of the different parts, and in the time of craftsman in installing the parts. Also, the more parts that are used, the more likely that the installation will be defective in that it will leak or have a poor appearance, unless considerable care is exercised by the installer. The use of several parts is especially disadvantageous for round or oval windows because many window installers are used to working only with parts having straight sides. A window assembly which used a minimum number of parts which were of simple construction and which could be installed in a simple manner, could lower the cost and increase the average quality of window installations.